1. Field of the Invention
The present invention relates to a helical antenna, and more particularly to characteristic improvement of a helical antenna which resonates at a plurality of frequencies.
2. Description of the Related Art
A portable radio apparatus is provided with an antenna at a top of a case, and by means of the antenna, a radio wave is radiated and caught to perform transmission-reception operation. Some portable telephone is provided with a helical antenna in which a protrusion amount of the antenna from the case is reduced. Other portable radio apparatus in recent years correspond to a plurality of radio communication systems, and therefore the antenna needs to have the characteristic of resonating in a plurality of frequency bands.
Consequently, as shown in FIG. 1, a helical antenna in which wire windings with different winding pitches are connected in series and placed at a top and bottom is proposed. In this helical antenna, conductive wire 3 wound around is housed in case 5 and feeder line 4 is drawn out of a lower end of case 5. In case 5, conductive wire 3 wound around forms wide pitch portion 7 at a lower side from pitch changing portion 6 and forms narrow pitch portion 8 at an upper side. The helical antenna has two resonant frequencies, that is, a first resonant frequency (fL) and a second resonant frequency (fH), which is higher than the first resonant frequency (fL).
An equivalent circuit of the conventional antenna will be described with reference to FIGS. 2A and 2B. FIG. 2A shows a schematic view of the conventional helical antenna, and FIG. 2B shows the equivalent circuit thereof.
Wide pitch portion 7 and narrow pitch portion 8 of the helical antenna constitute a rod antenna. Since adjacent conductive wires (windings) 3 are placed in close proximity in narrow pitch portion 8, a capacitor is formed between winding wires 3. Therefore, parallel resonant circuit 12 which is the result of connecting the capacitor and an inductance by the winding in parallel is formed near pitch changing portion 6 at which wide pitch portion 7 and narrow pitch portion 8 are switched, and this works as a trap for selectively passing or blocking a specified frequency. This parallel resonant circuit 12 is constructed to resonate at a second resonance frequency (fH). Thus, the parallel resonant circuit 12 has high impedance at the second resonance frequency (fH), and resonates at wide pitch portion 7 at a lower side from pitch changing portion 6. On the other hand, parallel resonant circuit 12 has low impedance at the first resonance frequency (fL), and resonates in the total length of wide pitch portion 7 and narrow pitch portion 8. As described above, an antenna tuning to two frequencies (fL, fH) is formed by the action of parallel resonant circuit 12 formed at narrow pitch portion 8.
FIG. 3 shows the frequency characteristic of a voltage standing wave ratio (VSWR) in the conventional helical antenna shown in FIG. 1. In this helical antenna, resonance occurs at a frequency (fL) near 900 MHz and a frequency (fH) near 1800 MHz, and transmission and reception of the radio waves can be made at two frequencies.
In the conventional helical antenna, a winding pitch and the number of windings are changed to adjust the characteristic of an antenna. For example, if the winding pitch is widened, the band width at the resonance frequency is widened, but if the winding pitch is changed without changing the entire length of the antenna, the number of windings is changed and a electric length of the antenna element (radiation element) is changed, thus changing the resonance frequency.
In concrete, the winding pitch in the narrow pitch portion is uniquely determined by the condition to form a capacitance component with which the parallel resonant circuit resonates at the second resonance frequency (fH) Thus, it is difficult to change parameters (the winding pitch, the number of windings) in the narrow pitch portion while the second resonance frequency (fH) and the resonance frequency of the parallel resonant circuit 12 are kept constant. Namely, even if the characteristics of the antenna is to be improved at the first resonance frequency (fL), the degree of freedom of electrical design of the antenna at the first resonance frequency (fL) is small, and there is the problem that improvement of the antenna characteristics such as VSWR, band width, radiation efficiency and the like at the first resonance frequency (fL) is difficult.
If the characteristics of the antenna are designed to improve without especially changing the outer dimension (total length) and the resonance frequency of the antenna, the winding pitches in wide pitch portion 7 and narrow pitch portion 8 change in relation to each other, and therefore the resonance frequency of parallel resonant circuit 12 cannot be changed. In other words, the design of the winding pitch of narrow pitch portion 8, the number of windings and the like cannot be changed. Therefore, it is difficult to improve the antenna characteristics.
The object of the present invention is to provide a helical antenna which facilitates the improvement of the antenna characteristics in the helical antenna having a plurality of resonance frequency.
The object of the present invention is achieved by a helical antenna comprising: a radiation element which is formed by winding or folding back an electric conductor; and a tip end element which is extended from the radiation element and placed in close proximity of the radiation element; the radiation element comprising: a first pitch portion which is connected to an feeding point and in which the electric conductor is wound around or folded back at a first winding pitch, and a second pitch portion which is connected to the first pitch portion and in which the electric conductor is wound around or folded back at a second winding pitch different from the first winding pitch.
In this helical antenna, the second pitch portion and the tip end element form a capacitance component (capacitor) to form frequency selection means at an intermediate portion of the radiation element, thus facilitating improvement in the antenna characteristics for each of a plurality of resonance frequencies.
The object of the present invention is also achieved by a helical antenna comprising: a radiation element which is formed by winding or folding back an electric conductor; said radiation element comprising: a first pitch portion which is connected to a feeding point and in which the electric conductor is wound around or folded back at a first winding pitch; a second pitch portion which is connected to the first pitch portion and in which the electric conductor is wound around or folded back at a second winding pitch narrower than the first pitch; and a third pitch portion which is connected to the second pitch portion and in which the electric conductor is wound around or folded back at a third winding pitch wider than the second winding pitch.
In this helical antenna, a capacitance component (capacitor) is formed by the second pitch portion to form frequency selection means, and at the low resonance frequency, the third pitch portion functions as an radiation element, thus facilitating improvement of the antenna characteristics for each of a plurality of resonance frequencies.